1. Field of the Invention
The present invention relates to a method of making an outer cannula for an intravenous cannula wherein the end of a previously formed tube of a shape memory resin such as a polyurethane resin is formed in a mold.
2. Description of the Prior Art
A catheter (herein referred to as an "intravenous cannula") is widely used for transfusing blood or liquid medicines. The intravenous cannula is maintained in the blood vessel of a patient for a time necessary for medical treatment. Generally, an intravenous cannula comprises an outer cannula made of a thermoplastic resin and an inner cannula made of stainless steel. The inner cannula is removed from a patient, after having been inserted with the outer cannula into the patient. Subsequently, a blood transfusion set or infusion set or the like is connected with the outer cannula remaining in the patient's body.
The distal end of an outer cannula has a thin peripheral wall such that the difference in outside diameters between the outer cannula and the inner cannula is minimized for a smoother piercing of the skin of a patient. The materials used to form an outer cannula include polytetrafluoroethylene (PTFE), polyurethane, polyethylene and the like.
In some cases, an outer cannula with a thin end is injection molded in its entirety. In other cases, one end of a previously prepared thin tube is mechanically sharpened using a machining apparatus.
Prior art methods of forming the thin end of an outer cannula are somewhat disadvantageous, particularly when a shape memory polyurethane resin is used. It has been difficult for an injection molding process to produce a thin outer cannula in a rapid and efficient manner from such a resin. An outer cannula is likely to be damaged at its thin end when and after it is ejected from a mold. Also, it is not easy to insert a cannula into the outer cannula. A separate process must be used and an automatic production system is not possible. Thus, despite the use of expensive molds, a labor and time consuming operation has been necessary to handle a molded cannula. Inner cannulas which vary greatly in diameter will cause significant variations in engagement strength when fitted in the respective outer cannula. In addition, except for the hot runner type of mold, injection molds generally produce a significant amount of waste, causing a remarkable loss in material and further increasing manufacturing costs. On the other hand, the mechanical sharpening of ends of previously produced tubes will inevitably cause them to be electrostatically charged during the process so that dust and chips will stick to the outer surface of the tubes.